Save log.tag as well while running test

Bug: 379667769
Test: setprop log.tag aaaa
setprop persist.log.tag bbbb
run `liblog_device_preparer.sh setup` and check the two sysprops are
reset.
run `liblog_device_preparer.sh teardown` and check the two sysprops are
restored.

Change-Id: I99bc8826cd7c3d1f55b5e6b6a3d0a6276af4e3d8